This application is related to Japanese Patent Application No. HEI 11 (1999)-230204 filed on Aug. 17, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an optical recording medium from which information is reproduced by use of light or near field light and a reproducing method therefor.
2. Description of Related Art
In recent years, optical memories typified by DVDs (digital-versatile-disks) and magneto-optical disks have been becoming remarkably high density. Recording density over several gigabits per square inch has already been realized. However, mass-storage devices with further high density are still demanded as storage devices for so-called home servers intended to store moving pictures and others.
In the optical memories, the recording density is basically determined by the diameter of a light spot (more precisely, a temperature distribution formed by a light spot). Accordingly, with the intention of forming a smaller light spot (the light spot is defined as an area having a light intensity of 1/e2 or higher), blue-violet lasers are being developed for shortening the wavelength of light sources and the NA (numerical aperture) is being enlarged by SILs (solid-immersion-lenses).
On the other hand, besides such techniques for reducing the light spot diameter, a super resolution technique using a mask layer is now being vigorously studied with a view to recording on and reproducing from a bit smaller than the light spot.
The super resolution technique using a mask layer is a technique of reproducing a bit smaller than the light spot without cross talk by providing an aperture smaller than the light spot with use of a temperature distribution or a light amount distribution formed by light irradiation of the mask layer. For example, Japanese Unexamined Patent Publication No. HEI 8(1996)-7333 discloses xe2x80x9can optical information medium using a mask layer which changes its reflectance to reproduction light by being heated to or over a transition temperature, which is within the range of 200 to 450xc2x0 C., by light irradiation and caused thereby to make a crystal-to-crystal transition.xe2x80x9d In addition to that, the publication also states that a breaching layer whose light transmittance increases with an increase in the intensity of incident light and mask layers using non-linear optical materials such as glass and the like, organic films and metals with low-melting temperature.
Also proposed is a high-density recording with near field light using a mask layer. For example, according to Applied Physics Letter, Vol.73, No.15, (1998), pp.2078 to 2080, reproduction with help of near field light can be realized by using an antimony film as a mask layer and forming an aperture smaller than a light spot.
In signal reproduction by the super resolution technique using such mask layers, it is required to satisfy high reliability: (1) degradation does not occur when the aperture is formed in a reproducing process; (2) there are no limit to the number of repetitions, and the like. Accordingly, it is important to decide what principle should be used for forming the aperture on the mask layer.
However, the mask layers according to the above-explained conventional techniques have problems in reliability such as (1) degradation by the reproducing process owing to high aperture-forming temperature, (2) a limited number of repetitions owing to a phase change involving melting or owing to the use of an organic film, and the like. Further, as the size of the aperture decreases, a signal to noise ratio (S/N) becomes worse. Especially, in the case where the near field light is utilized, the S/N declines remarkably so that it becomes hard to reproduce signals.
In view of the above-described problems, an object of the present invention is to provide an optical recording medium and a reproducing method therefor which allow signals to be reproduced from a bit smaller than the light spot with a good S/N and allow high-density recording and reproduction without reducing the size of the light spot.
The inventors of the present invention have paid attention not only to an aperture formed at a central part in the light spot but also to its peripheral region (a peripheral part in the light spot) for realizing an optical recording medium and its reproducing method which allow super resolution reproducing with a superior S/N.
For achieving the above-mentioned object, the present invention provides an optical recording medium comprising a recording layer and a mask layer on a substrate, wherein the mask layer is formed of a material whose light transmittance is decreased by light irradiation and is increased by heating.
The invention is also characterized in that, in the above-described optical recording medium, the mask layer has a threshold light transmittance with respect to the amount of light applied to the mask layer and the light transmittance at a central part in a light spot formed by the applied light is set to be higher than the light transmittance at a peripheral part in the light spot and lower than the light transmittance at a part of the mask layer which is not irradiated with light.
The invention is also characterized in that, in the above-described optical recording medium, the mask layer has a threshold electrical resistivity with respect to the amount of light applied to the mask layer and the electrical resistivity at the central part in the light spot formed by the applied light is set to be higher than the electrical resistivity at the peripheral part in the light spot and lower than the electrical resistivity at a part of the mask layer which is not irradiated with light.
Further, the invention is characterized in that a change in the light transmittance and the electrical resistivity of the mask layer is induced by a metal-to-insulator transition.
In another aspect, the present invention provides a reproducing method for an optical recording medium comprising, when information is reproduced with irradiating with light an optical recording medium provided with a recording layer and a mask layer on a substrate, the mask layer having a light transmittance which is decreased by light irradiation and increased by heating, setting the amount of light in a light spot formed by light applied to the mask layer to or above a threshold amount of light at which the mask layer transits from an insulator to a metal and setting the temperature at a central part in the light spot to or above a threshold temperature at which the mask layer transits from a metal to an insulator.
The present invention also provides a reproducing method for an optical recording medium comprising, when information is reproduced with irradiating with near field light an optical recording medium provided with a recording layer and a mask layer on a substrate, the mask layer having a light transmittance which is decreased by light irradiation and increased by heating, enhancing the near field light generated via an aperture at a central part of a light spot by means of an annular region having a low electrical resistivity formed around the aperture.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.